The present invention concerns the machining of sheets, or even of plates (comparable to sheets in the context of this invention), with a view to obtaining nonparallel faces on said sheets.
In metallurgy, rough sheets are usually obtained by rolling. A rolling operation results in a difference in rate of advance of the sheet between upstream and downstream in the rolling operation, which is manifested by an increase in length of the sheet after rolling. Fundamentally, in an operation as described above, the dimensions of the sheet are changed, while keeping a rectangular profile (section perpendicular to the direction of movement). The product obtained comes with generally parallel faces, with a certain level of tolerance for the flatness of each of the faces and for the thickness between faces. If it is desired to hold the flatness and/or thickness within close tolerances, the sheets generally have to be rectified.
Furthermore, starting with a standard sheet, it is difficult to obtain profiles with nonparallel faces by rolling. If, for example, the respective axes of the rollers are not parallel, the volume of material treated per unit of width (dimension parallel to the axis of the rollers) is no longer constant. As a result, the rate of advance tends to be greater in the places rolled to lesser thickness. That produces very sizable internal stresses in the rolled sheet and, therefore, very great difficulty in obtaining a rolled product without curvature. There again, the sheets often have to be straightened to obtain the desired profile.